This invention is related to visual display devices. It is particularly directed toward an improved gas discharge display for use in flat panel television, alpha-numeric displays and the like.
Gas discharge displays generally include a matrix of rows and columns of individual gas discharge cells. Typically, there are corresponding matrices and rows of columns of wire conductors which intersect at the individual cell locations. Each cell has at least an anode conductor and a cathode conductor between which a low pressure atmosphere of a gas, such as a noble gas and/or mercury vapor, is maintained. When a sufficient potential is applied between an anode conductor and a cathode conductor, a gas discharge is developed at the cell which is located at their intersection. A visible "cathode glow" is then established near the cathode and, in some applications, it is that glow which is used as the visible light output of the cell.
Between the anode conductor and the cathode glow a plasma or "positive column" exists which includes energetic electrons, excited atoms and ions. These particles are continuously recombining, regenerating and colliding. The collision of an energetic electron with a gas atom produces a high energy state in the atom's electron shell which decays to a lower energy state, thereby causing an emission of radiation from the atom and from the positive column. The gas constituents and the operating parameters of a cell may be chosen such that the radiation emanating from a positive column is in the UV (ultraviolet) spectrum. The UV radiation may then be converted into visible light of a predetermined color by directing the UV radiation onto a UV-excitable phosphor coated on one or more of the cell walls.
When excited by the UV radiation, the phosphor coating emits visible light of the predetermined color. It is with this mode of generating visible light from a gas discharge cell that this invention is primarily concerned, although certain aspects of it are also applicable to applications utilizing only the cathode glow as the source of light output, as well as to applications where light is produced by direct electron excitation of a phosphor.
In the past, gas discharge displays have suffered from a number of problems, one of which has been the high voltage required to drive the anode conductors to establish a discharge in selected cells. In typical commercial applications, anode conductors are driven with a potential of several hundred volts. Generally, the potential applied to such anode conductors is in the form of a time-varying signal which corresponds to video information, so that many high voltage video drivers are required to drive the individual anode conductors. In a television application, a typical gas discharge panel will have several hundred anode conductors, each driven by a video driver. To implement the circuitry required for such a system would be prohibitively expensive unless the circuitry could be realized in integrated circuit (IC) form. Since present IC technology is, for the most part, limited to the production of video circuitry capable of handling less than 150 volts, the video drivers can probably not be profitably integrated at this time. Instead, a discrete high voltage video driver is probably required for each anode conductor. A display incorporating discrete video drivers for each anode conductor would obviously be too expensive for consumer applications.
Another problem associated with prior art gas discharge displays is that, when it is desirable to draw a relatively high current through a cell in order to produce a high light output therefrom, the cathode of such cells often operate in the "abnormal mode" wherein the voltage across the cathode fall region increases. When a cathode is operating abnormally, the anode voltage of that cell must be increased in order to sustain a high current discharge therein. This increase in the anode voltage causes a concurrent increase in the power consumption of the cell and a resultant decrease in the operating efficiency of the cell; i.e., a decrease in the lumens output per watt input of the cell. In addition, the required increase in the anode voltage increases the possibility of spurious discharges occurring between the anode and other elements of the cell or adjacent cells, a highly undesirable condition.
The problem of operating a cathode in the abnormal region is aggravated when the cathode is used in a cell having a very low gas pressure. Although gas discharge cells operate more efficiently at such low pressures (see U.S. application Ser. No. 436,249, now U.S. Pat. No. 3,899,636 at least at low current levels, the cathode is more likely to go abnormal when higher cell currents are required. It is very desirable, therefore, to have an array of cells whose cathodes are capable of operating in low pressure atmospheres without going abnormal when they are required to deliver a relatively high level current.
An additional aspect of gas discharge display design with which this invention is concerned is the ability of the panel to scan sequentially and automatically from row to row in a way which is synchronized to a scanning video input signal such as a television signal. Without some sort of automatic scanning incorporated in the display, each cell would require that both its anode and its cathode be energized by separate anode and cathode drivers which can be sequentially switched from an "off" mode to an "on" mode. To provide each cell with separate switchable anode and cathode drivers would require undue circuit complexity and expense.
Although some gas discharge panels do feature a mode of automatic scanning for the cathode conductors, no such panel has been implemented in a way which provides both automatic scanning and cathodes capable of efficient high current density operation at low gas pressures.
A final problem which is common to many existing and proposed gas discharge display systems is the structural complexity of the panels themselves. In the commercial world where panels must be assembled in the hundreds or thousands, a requirement that multiple layers or sheets be stacked one on top of another in registration may be prohibitively expensive. A preferred panel has a minimum number of layers which must be aligned and has a system of conductors which easily mate with the various cell layers. In addition, a gas discharge display suitable for consumer applications should be easily fabricated in a number of viewing sizes.
Although all the above-mentioned problems are not found in every gas discharge panel, most suffer from one or more of them. A gas discharge panel, especially one intended for consumer use, should meet each such problem if it is to mature into commercial reality.